80 PLANT GROWTH 



of carbon dioxide used, diffuses from the cell to the inter- 

 cellular spaces and then through the stomata to the atmos- 

 phere. 



The sugar is truly the food material of the living world. 

 All animals, and all fungi, as well as green plants, depend on 

 it as their basic food. The plant can convert the sugar into 

 starch for storage, or it may use it to make the cellulose 

 material of which the cell walls are made, and in trees become 

 lignified to make all the wood of the forest. By the addition 

 of small amounts of nitrogen and a few other elements, the 

 proteins are made from sugar (Chapter 23). Fats are simi- 

 lar in structure to sugars, but many of the details of their 

 synthesis, from sugars, remains a mystery. 



About two hundred years ago it was believed that plants 

 grew from air and water. It seems nearly true even today 

 because many plants such as lettuce, apples, and cucumbers, 

 to mention only a few, consist of more than 90 per cent water 

 and about 9 per cent of the products of photosynthesis. All 

 plants consist largely of water and sugar or its products. 

 The corn studied by Miller was about 98.5 per cent water and 

 air. His analysis showed about 69 per cent water, 29 per cent 

 material of photosynthesis, and about 1.5 per cent of the ele- 

 ments taken from the soil. Water is so often a limiting factor 

 in growth that we cannot stress too much the importance of 

 having a favorable water supply and conditions favorable 

 to photosynthesis. 



It is difficult to measure the rate of photosynthesis or to 

 measure the amount of sugar made because the plant uses it 

 continuously. The best estimates are between a half an 

 ounce and an ounce per square yard of leaf surface per day 

 under favorable conditions. This seems like a small amount 

 but if one considers the number of days it takes place and 

 tries to measure the number of square yards of leaf surface 

 in the lawn or on a tree, which is much more than the surface 

 area of the ground covered, it becomes more evident that the 



